A wide variety of steam cookers are known. While pressure cookers have long been used in homes and restaurants, the risks and extra costs of containing steam under high pressure have led to the growth of slow steam cookers that use steam to deliver heat to the food, but at a pressure that is typically just above atmosphere (e.g., a few inches of water). These slow steam cookers are commonly characterized as “pressureless” or “atmospheric.”
Most conventional slow steam cookers have steam generators, typically boilers external to the cooking compartment that use electrical resistance or gas heaters that produce steam from a water supply.
A problem associated with many known pressureless steam cookers is excessive water consumption. Steam is drained from the cooking compartment through a drain pipe leading to a waste line. Many local codes regulate the operation of steam cookers and other water heating devices, barring draining water to a municipal waste line if its temperature is above 140° F. To comply with these regulations, the present standard practice is to spray cooling water into the exit steam to reduce the temperature of the condensate and steam fed into the drain line to an acceptable, lower level. This gives rise to water consumption in commercial steam cookers can exceed 50 gallons per hour, regardless of the quantity of food being cooked. A ten-pan, twin compartment, twin-generator steam cooker manufactured by Crown Food Service Corp. and described in U.S. Pat. No. 6,000,392 uses more than 100 gallons of water per hour to cool the drained steam and condensate.
In addition to poor water consumption efficiency, the thermal efficiency of a conventional pressureless steam cooker is not high. Regardless of the quantity of food being steamed in the cooking compartment, if the flow of steam into the compartment from a steam generator is continuous there is another significant loss of thermal energy In the initial phase of cooking a food product that is at room temperature, or is in a frozen state, the volume of steam required at the outset of cooking must be high. But when the food produced is thereafter in a heated condition, less steam is then required to complete the cooking operation. Many conventional pressureless steam cookers do not take into account the varying demand for steam in the course of a cooking operation, which reduces their efficiency.
Conventional pressureless steamers make it possible for a chef to operate the steamer as he does a conventional oven, opening its door to inspect or work with the foods, or to remove a cooking pan or pans from the steamer when the food is cooked. However, as steam is supplied to a cooking compartment by a boiler under pressure, when the door is opened to remove the cooked food, steam is then released to the atmosphere. The operator should exercise care in positioning himself away from the cooking compartment when opening its door.
The opening and closing of the oven door also produces a loss of steam, fluctuations in steam pressure, and introduce cold air into the compartment. The efficiency and the quality of operation of the steam cooker is dependent upon the degree to which the temperature and pressure of the steam within the cavity can be maintained at or near a preselected optimal value, or within an optimal range of values, despite variations due to the amount, type and temperature of the food being cooked and fluctuations in the steam temperature, movement, and pressure through the cooking cycle, and as the cooking compartment door is opened and closed.
The build-up of lime on the electric heater coils or the tubes of the steam generator because of the mineral content of the water being boiled also limits the thermal efficiency of known steamers. The resultant lime coating acts as a thermal insulating layer, thereby causing high-watt density electrical resistance heaters which are immersed in water to overheat and self-destruct. Where water is boiled by a gas-fired water-tubes boiler, a lime coating collects on the interior walls of the boiler tubes. The conventional practice is to remove the lime coating on the heater coils or the boiler tubes by means of a chemical solvent. But before the steam generator can be put back into service, the solvent must be completely flushed away, otherwise the solvent will contaminate the steam. The formation of lime deposits both limits energy efficiency and reduces the availability and convenience of use of slow steam cookers.
U.S. Pat. Nos. 5,549,038 and 5,631,033 to Kolvites and assigned to Market Forge Industries, Inc. and implemented in the six-pan steam cooker sold by Market Forge under the trade designation “STEAM TECH PLUS,” describe solutions to some of these problems. Periodically, on demand, this cooker can draw water from a supply, heat the water in a boiler to produce steam, circulate the steam in a cooking chamber around the food to cook it and/or keep it warm until served, and then direct the steam and condensed water to a drain. The boiler is mounted on the side of an associated oven in direct fluid communication with that cooking chamber.
The Kolvites steamer modulates the power supplied to the boiler, and therefore the heat produced, to accommodate variations in the quantity of food, its temperature, and its surface area. A small amount of room temperature food will be cooked quickly with the steam generator powered. Continued heating will generate a dangerous overpressure and overcook the food. In the Kolvites steamer, a pressure-sensitive switch is connected in series in the power supply line. The switch responds to the steam pressure in an outlet conduit from the cooking compartment. To control an out-rush of steam when the cooking compartment door is opened, the Kolvites cooker uses a switch responsive to an opening of the door opens a valve in a fresh water supply line. The resulting cold water spray quenches (or “blows down”) steam in the steam generator, creating a pressure drop there that draws steam in the cooking compartment back toward the boiler. This also conserves the steam, saving water and energy. A cold water spray is also used to cool a mechanical steam trap to open the steam outlet line to atmosphere when the door is opened. The Kolvites drain line from the cooking compartment terminates in, and is water-sealed to, a water filled tempering tank located below the cooking compartment. On start-up, the steam trap is cool and open to provide a path for cold air displaced by the new steam to escape to atmosphere.
U.S. Pat. No. 6,000,392 discloses a slow steam cooker with a steam generator for each cooking compartment that has a water tank adjacent to the cooking compartment heated by gas-fired, open-ended tubes immersed in the tank The steam produced is conducted to the cooking compartment by a narrow conduit.
These known pressureless steamers offer many advantages, such as rapid, efficient cooking of large volumes of food, including frozen food. Their disadvantages include one or more of: a relatively large water usage; attendant high power requirements to heat the water; and burdensome maintenance requirements such as daily, monthly and annual cleanings to remove scaling (“de-liming”) on heating coils, tubes and other components produced by the boiling, as well as to remove residue from the cooking process itself. Also, as noted above, steam and hot water drained from these slow steam cookers must be cooled before it is drained to public waste systems.
It is therefore a principal object of this invention to provide a pressureless steamer that has improved energy usage and water consumption efficiencies.
Another object of the invention is to provide a pressureless steamer with the foregoing advantages that is gas fired.
Another object of the invention is to provide a steamer with the foregoing advantages that is freestanding, has high (e.g. twin oven, ten pan) cooking capacity, and is nevertheless compact and highly integrated.
Still another object of the invention is to provide the foregoing advantages with a ease of use, including deliming, as well as safe and reliable operation.
Yet another object of this invention is to provide an atmospheric twin generator steamer that is sufficiently efficient in its cooking energy usage to meet current US Government Energy Star standards.